AN824

Vishay Siliconix

Document Number: 71990

06-Jan-03

www.vishay.com

3

Board pad design. The landing-pad size for MICRO FOOT

products is determined by the bump pitch as shown in Table 3.

The pad pattern is circular to ensure a symmetric,

barrel-shaped solder bump.

TABLE 3

Dimensions of Copper Pad and Solder Mask

Opening in PCB and Stencil Aperture

Pitch

Copper Pad

Solder Mask

Opening

Stencil

Aperture

0.80 mm

0.30 " 0.01 mm

0.41 " 0.01 mm

0.33 " 0.01 mm

in ciircle aperture

0.50 mm

0.17 " 0.01 mm

0.27 " 0.01 mm

0.30 " 0.01 mm

in square aperture

ASSEMBLY PROCESS

MICRO FOOT products’ surface-mount-assembly operations

include solder paste printing, component placement, and

solder reflow as shown in the process flow chart (Figure 5).

FIGURE 5. SMT Assembly Process Flow

Stencil Design

IIncoming Tape and Reel Inspection

Solder Paste Printing

Chip Placement

Reflow

Solder Joint Inspection

Pack and Ship

Stencil design. Stencil design is the key to ensuring

maximum solder paste deposition without compromising the

assembly yield from solder joint defects (such as bridging and

extraneous solder spheres). The stencil aperture is dependent

on the copper pad size, the solder mask opening, and the

quantity of solder paste.

In MICRO FOOT products, the stencil is 0.125-mm (5-mils)

thick. The recommended apertures are shown in Table 3 and

are fabricated by laser cut.

Solder-paste printing. The solder-paste printing process

involves transferring solder paste through pre-defined

apertures via application of pressure.

In MICRO FOOT products, the solder paste used is UP78

No-clean eutectic 63 Sn/37Pb type3 or finer solder paste.

Chip pick-and-placement. MICRO FOOT products can be

picked and placed with standard pick-and-place equipment.

The recommended pick-and-place force is 150 g. Though the

part will self-center during solder reflow, the maximum

placement offset is 0.02 mm.

Reflow Process. MICRO FOOT products can be assembled

using standard SMT reflow processes. Similar to any other

package, the thermal profile at specific board locations must

be determined. Nitrogen purge is recommended during reflow

operation. Figure 6 shows a typical reflow profile.

0

50

100

150

200

250

0

100

200

300

400

Thermal Profile

Time (Seconds

FIGURE 6. Reflow Profile

PCB REWORK

To replace MICRO FOOT products on PCB, the rework

procedure is much like the rework process for a standard BGA

or CSP, as long as the rework process duplicates the original

reflow profile. The key steps are as follows:

1. Remove the MICRO FOOT device using a convection

nozzle to create localized heating similar to the original

reflow profile. Preheat from the bottom.

2. Once the nozzle temperature is +190_C, use tweezers to

remove the part to be replaced.

3. Resurface the pads using a temperature-controlled

soldering iron.

4. Apply gel flux to the pad.

5. Use a vacuum needle pick-up tip to pick up the

replacement part, and use a placement jig to placed it

accurately.

6. Reflow the part using the same convection nozzle, and

preheat from the bottom, matching the original reflow

profile.